Field of the Invention
Described herein are devices and methods relating to surface mount devices, such as light emitting diode (LED) chips and components, including LED chips, which have recessed contact pads.
Description of the Related Art
Surface mount devices, such as LED-based light emitting devices, are increasingly being used in lighting/illumination applications. Semiconductor LEDs are widely known solid-state lighting elements that are capable of generating light upon application of voltage thereto. LEDs generally include a diode region having first and second opposing surfaces, and including therein an n-type layer, a p-type layer and a p-n junction. An anode contact ohmically contacts the p-type layer and a cathode contact ohmically contacts the n-type layer. In some cases, the diode region may be epitaxially formed on a substrate, such as a sapphire, silicon, silicon carbide, gallium arsenide, gallium nitride, etc., or growth substrate, but the completed device may have the substrate removed. The diode region may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride and/or gallium arsenide-based materials and/or from organic semiconductor-based materials. In other configurations, it may be possible for the device to never include a substrate, such as if grown or processed on a virtual wafer.
Submounts are generally used in LED devices to interpose an LED chip and a printed circuit board. The submount may change the contact configuration of the LED chip to be compatible with the pads of the printed circuit board. The submount may also be used to support a phosphor layer or an encapsulating dome that surrounds the LED chip. The submount may also provide other functionality. Thus, a submount may include a receiving element onto which an LED chip is mounted using conventional die-attach techniques, to interface the LED chip and a printed circuit board. A submount generally has a thickness of at least 100 μm, and in some embodiments at least 150 μm, and in other embodiments at least 200 μm, and generally includes traces (such as on ceramic panels) and/or leads (such as in a Plastic Leaded Chip Carrier (PLCC) package).
The color or wavelength emitted by an LED is largely dependent on the properties of the material from which it is generated, such as the bandgap of the active region. LEDs have been built to emit light in a range of colors in the visible spectrum including red, yellow, green, and blue. Other LEDs emit in the ultraviolet (UV) range of the electromagnetic spectrum. It is often desirable to incorporate phosphors into a LED to tailor the emission spectrum by converting all or a portion of the light from the LED before it is emitted as it passes through. For example, in some blue LEDs, a portion of the blue light is “downconverted” to yellow light. Thus, the LED emits a combination of blue and yellow light to generate a spectrum that appears white to the human eye. This is known as a blue-shifted yellow (BSY) LED device. As used herein, the term “phosphor” is used generically to indicate any photoluminescent material.
Because of the above issues, the application of a conversion layer to an LED chip is typically done at the package level after the LEDs have already been singulated and subsequently bonded to an electronic element, such as a PCB. However, applying a conversion material at the package level rather than the wafer level is a less efficient manufacturing process, as it is much easier and cost effective to coat multiple LED chips simultaneously at the wafer level.
It is desirable to complete as many steps as possible at the wafer level rather than at the package level, as it is more efficient to do so for manufacturing purposes. For surface mount devices, which have recessed contact pads in relation to the mounting surface of the device, these contacts to the mount surface at this time are being formed by adding gull wing type attachments to the contact pads at a package level. Otherwise, large amounts of metal paste are added to the mount surface in the hopes that during the mounting process this metal paste will enter the recessed areas and make contact with the contact pads. Unfortunately, this process can be unreliable, include low yields with regard to successful bonding, and can result in imbalanced devices or unbalanced amounts of material on each contact pad of the device. It would be desirable to utilize a more reliable and controlled process to create contacts, preferably at the wafer level, which could also increase manufacturing efficiencies.